In recent years, various solid oxide fuel devices comprising fuel cells capable of obtaining electrical power by generating electricity using fuel (hydrogen gas) and air, as well as auxiliary equipment for running such fuel cells, have been proposed as next-generation energy sources.
Japanese Unexamined Patent Application H. 7-307163 (Patent Document 1) sets forth a fuel cell device. In this fuel cell device, the electrical power generated is varied in response to load.
Here, referring to FIG. 15, we explain a power supply system utilizing a fuel cell. FIG. 15 shows an example of a conventional system for supplying electrical power to a residence using a fuel cell. In the system, electrical power consumed by a residence 200 is supplied by fuel cell 202 and grid power 204. Normally the maximum consumed power consumed by residences is larger than the maximum rated power generatable by fuel cell 202, therefore even in a residence 200 utilizing a fuel cell 202, that insufficiency is made up for by grid power 204, and electrical power is supplied to the residence from fuel cell 202 and grid power 204. Furthermore, even in situations where the generating capacity of fuel cell 202 is below the maximum rated power for a residence, a portion of the electrical power consumption of residence 200 is normally supplied from the grid power 204 in order to prevent reverse current flow of generated power to current power 204.
Grid power 204 is power fed from a transmission line to an electrical distribution panel inside a residence, and is purchased power. In other words, the total of the electrical power generated by fuel cell 202 and grid power 204 corresponds to the power consumed by residence 200. Fuel cell 202 obtains a monitor signal from power demand detector 206 of the electrical power purchased by residence 200, and based on this it varies the power generated by fuel cell 202. I.e., fuel cell 202 determines a base current Ii expressing the current which fuel cell 202 should produce based on the monitor signal obtained from power demand detector 206, and controls the amount of fuel, etc. supplied to fuel cell module 208 to enable production of this base current Ii. Also, base current Ii is set at or below a value corresponding to the maximum rated power of fuel cell 202, regardless of the power consumed by residence 200.
The fuel cell module 208 built into fuel cell 202 generally has an extremely slow response, making it difficult to change generated power to follow changes in power consumed by residence 200. Therefore the base current Ii signal which instructs an electrical generation amount to fuel cell module 208 is determined by applying a filter 210 which performs integration or the like on the monitor signal, so that it changes extremely gradually compared to the change in power consumption.
Fuel cell 202 supplies fuel cell module 208 with an amount of fuel proportional to base current Ii so that fuel cell module 208 has the capacity to produce the base current Ii. At the same time, inverter 212 extracts a DC extracted current Ic from fuel cell module 208 and converts this to AC and supplies it to residence 200. The actual extracted current Ic which inverter 212 extracts from fuel cell module 208 is at all times set at or below the value of base current Ii, and does not exceed the generating capacity of fuel cell module 208. If a current equal to or greater than the generating capacity corresponding to the fuel supply amount, etc. determined based on base current Ii is extracted from fuel cell module 208, then there is a risk that fuel depletion in fuel cells within the fuel cell module 208 will occur, dramatically shortening the life span of the fuel cells and damaging the fuel cells.
At the same time, because of sharp fluctuations in the power consumed by residence 200, when consumed power suddenly drops, the residence 200 power consumption drops further than the power corresponding to base current Ii, which is slowly varied.
In the fuel cell generating apparatus set forth in Japanese Published Unexamined Patent Application H.07-307163, an inverter required to respond to suddenly fluctuating power consumption and a fuel cell module incapable of following sudden changes in generated power are controlled by a single control means.
Japanese Unexamined Patent Application H7-307163